selftests: forwarding: Add suppport to create veth interfaces

For tests using veth interfaces, the test infrastructure can create
the netdevs if they do not exist. Arguably this is a preferred approach
since the tests require p$N and p$(N+1) to be pairs.

Signed-off-by: David Ahern <dsahern@gmail.com>
Reviewed-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>